| description |
Leaves of almost all C4 lineages separate the reactions of photosynthesis into mesophyll (M) and bundle sheath (BS) cells. The extent to which molecular signatures of M and BS cells from independent C4 lineages resemble each other is not known. To provide insight into this, we rolled leaves of S. viridis leaves to release M cell sap followed by mechanical disruption to isolate BS cells. Deep sequencing of mRNA isolated from these samples revealed a high correlation between the relative abundance of mRNAs encoding proteins of the core C4 pathway in M and BS cells (r = 0.89) indicating significant convergence in mRNA expression between S. viridis and the evolutionarily independent C4 lineage maize. We also found that the vast majority of genes encoding proteins of the C4 cycle in S. viridis are syntenic to homologues used by maize. Of the 5,049 genes that were more abundantly transcribed in the M of S. viridis, 37% shared an homologue that was also M specific in maize, while in the BS 39% of the 4,631 genes had a BS specific homologue in maize. Ten categories of gene ontology term were enriched in both species, of which seven were enriched within the same cell type. In both lineages 122 and 212 homologous transcription factors were preferentially expressed in the M and BS respectively. Eighteen shared chloroplastic regulators were identified including, GLK1, GLK2, SIG2, SIG3, and CRB which are known to regulate the light-dependent reactions of photosynthesis. Fourteen of these eighteen regulatory genes were syntenic homologues. Taken together these data provide the first estimate for the extent of convergence in gene expression of both structural and regulatory genes in the M and BS cells of C4 grasses. Furthermore, the repeated recruitment of syntenic homologues from large gene families strongly implies that parallel evolution of both structural genes and also trans-factors underpins the polyphyletic evolution of this highly complex trait in the monocotyledons. |